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Acochlidium fijiiensisis belongs to a group of opisthobranchs - the Acochlidea - which have freshwater species as well as marine species. Most acochlidian species are small worm-like marine slugs found living between sandgrains - part of what we call the interstitial fauna. However some have invaded brackish water and others freshwater. One small species, Tantulum elegans, is found in a mountain swamp on the Caribbean island of St Vincent. However most of the others we know of are much larger than usual, up to 3 cm in length, and live in rivers and streams on several tropical Pacific islands [see message #16865 for a list].

Acochlidium fijiiensis is small and shell-less, but looks snail-like because of the large recurved visceral hump. It can grow up to 19 mm in length. It is translucent yellowish with irregular brownish-black bands across the body. Like many acochlideans small calcareous spicules can be seen in the skin of the visceral hump and some are also present in the foot. Despite searches in other parts of Fiji. this species has only been recorded from the Nasekawa River (Vanua Levu) and Lami River (Viti Levu).

From their studies between 1983 and 1988, Haynes and Kenchington (1991) found this species lives under stones and rocks in shallow parts of streams (6 - 14 cm deep) influenced by the tide, but upstream of any sea water. They become reproductively mature in July when their visceral hump expands to accommodate the enlarging eggs.